30TH INTERNATIONAL COSMIC RAY CONFERENCE

The Change in Cosmic Ray Intensity Variation with the Solar Wind Velocity(Using GRAPES-3 muon narrow angle telescopes and Kiel neutron monitor)
H. KOJIMA2, Y. HAYASHI1, K. HAYASHI1, N. ITO1, S. KAWAKAMI1, T. MATSUYAMA1, M. MINAMINO1, T.
NONAKA1, T. OKUDA1, S. OGIO1, A. OSHIMA1, N.SHIMIZU1, S.K. GUPTA3, A. IYER3, P. JAGADEESAN3, A.
JAIN3, S. KARTHIKEYAN3, P.K. MOHANTY3, S.D. MORRIS3, P.K. NAYAK3, B.S. RAO3, K.C. RAVINDRAN3, K.
SIVAPRASAD3, H. TANAKA3, S.C. TONWAR3, S. SHIBATA4, I. MORISHITA51Graduate School of Science, Osaka City University
2
Nagoya Women’s University
3
Tata Institute of Fundamental Research
4
College of Engineering, Chubu University
5
Department of Information Management, Asahi University
hkojima@nagoya-wu.ac.jp
Abstract: GRAPES-3 experiment is situated at Ooty in southern India 76.7 East 11.4 North (degree).
Effective observation area of our muon telescopes is 560 m2. They are the largest detector in theworld of its kind. There were several reports that suggest the increase of the solar wind velocity suppresses the intensity of cosmic rays. But there are only a few which have studied this quantitatively.
We have analyzed the variation of daily mean of counting rate of low energy muons >~1GeV with solar wind velocity. Our muon telescope data are used together with Kiels’neutron monitor for 2000 to2005. These 6 years correspond to solar maximum to minimum. Periods for Forbush decreases havebeen removed from analysis to avoid unusual response of muons rate during those periods. In case ofmuons their intensity decreases with solar wind velocity -0.0013% per km/s with relative coefficientof 0.97. For neutron monitor it is -0.0032% per km/s with relative coefficient 0.96. These results suggest there is clear correlation between solar wind velocity and the intensity of cosmic rays. Significant changes in the slopes were seen between first 3 years data and later 3 years one.

Introduction
The diffusion coefficient for the propagation ofcosmic rays is required to describe the density ofcosmic rays which is penetrating into the solar
system from inter stellar space. According to theDiffusion and Convection theory, the intensity ofcosmic rays varies with the variation of solar
wind velocity. If we can clarify the relation between these two quantitatively, it would be useful
to obtain the diffusion coefficient for the propagation by observation.
There were several reports so far that cosmic rayintensity decreases with the increase of solar windvelocity (we call this phenomena as cosmic raysolar wind effect) since the beginning of the direct measurements with satellites (October 1963).
Those reports are mostly concerned with thecosmic ray intensity variations caused by highspeed solar wind. There are only a few works thatare done for the cosmic ray intensity variations
with the variations of solar wind velocity quantitatively. The main reason is the following. There
are two kind of intensity variations in the cosmic
rays, one is long term variation such as 11 yearsof solar cycle, other is short term variation likeForbush decrease. Other than these variations
there are some variation due to the Earth’s revolution of its own and around the Sun. These different causes cover up each other in the ground levelobservations. In case of solar wind, several variations are seen, such as high speed solar wind withsolar flare, 27 days cycle of solar revolution andsome several years variation. It is quite tough toisolate the genuine variation from those various
causes. The simple changes of cosmic rays andsolar wind velocity variations would not be able
to create successful explanation.

SHORT TITLE
Some reports suggest that direction of galactic
cosmic rays flowing into the solar system changes
its direction with solar magnetic dipole field. If
we can find the North-South direction effect on
cosmic ray-solar wind effect, it will be very useful to explain the cause of cosmic rays flow into
the solar system.
We are measuring muon’s directional intensity
variation from 1999 onwards. Our apparatus are
situated in southern India (76 deg 40 min Eastand 11 deg 23 min North, 2200 m a.s.l.) and consist of 16 independent modules, each module has
effective area of 35 m2 and total size of 560 m2.
This is the largest muon telescope in the world formuon measurement.
We have analyzed muon directional intensity
variation data for 6 years, 2000 to 2005. It covers
the duration of the maximum to minimum of solar
activity. The neutron monitor’s data from Kiel
were also analyzed for same period.
In order to remove various components whicharise from many causes described above we applied digital filtering of 27 days period on bothcosmic rays and solar wind velocity. We could
remove the long term contamination. The shortterm variation due to the Forbush decreases havebeen eliminated by removing the periods whichshow more than 2% decrease in the Kiel’s neutron
monitor.
The followings are the results from the analysis.

Analysis and Results
Analysis has been done by using 2000 to 2005 six
years data of both Kiel’s neutron monitor and
GRAPES-3’s muon directional telescopes. The
detail of our apparatus can be seen in articles.
Even though we can measure the rather detailed
feature in muon’s direction, we categorized thosemuon into 9 direction (see Fig.1).

Fig1. GRAPES3 muon narrow angle telescopes
observed directionsFig.2 and fig.3 show the variation of intensities ofmuons and neutrons and solar wind velocity.

Fig2. GRAPES3 muon (V) intensity and solar
wind velocity

Fig3. Kiel neutron intensity and solar wind velocity
One can see the long term intensity variation in
addition to the daily variation in both cosmic rays
intensity and solar wind velocity. The amount ofvariation is ~3% in case of muons intensity and~8% in case of neutrons for 6 year’s duration. In
case of solar wind velocity amplitude of~100km/s with ~3 years cycle can be seen. This
long term variation looks due to the part of 11
years solar activity. It is known that there weresome several years variation due to the appearance of coronal hole. In any case it seems to be
extremely difficult to extract the genuine cosmic

Fig4. GRAPES3 muon intensity V and solar
wind speed after high-pass filter is processed ofthe 27day period

30TH INTERNATIONAL COSMIC RAY CONFERENCE 2007

Fig5 ． Kiel neutron intensity and solar windspeed after high-pass filter is processed of the27day periodrays intensity solar wind effect from the raw data,
since there are quite a lot of contamination.
As you can see clearly in Fig.4 and Fig.5 the longterm variation are removed from both cosmic rays
intensity and solar wind velocity.
After processing with the filtering, the data aresorted by the solar wind velocity (daily mean) in
7 groups, v<-80km, -80km<v<-40km, 40km<v<0km, 0km<v<40km, 40km<v<80km,80km<v<120km, 120km<v. The average is calculated for each of these 7 groups and regression
analysis has been applied with solar wind velocity
variation as independent variable and cosmic rays

Fig8. Solar wind velocity
variation(x) andmuon(V) intensity variation(y)Fig9. Solar wind velocity variation (x) andmuon (S) intensity variation (y)intensity variation as dependent variable. The
results are shown in Fig.6 ~Fig.9.
One can see clear tendency that cosmic ray intensity variation decreases with the increase in solar
wind velocity. The slope for neutron is larger thanmuon’s slope. We split the 6 years data into twogroups of 3 years, 2000~2002, 2003~2005 andrepeat the same analysis. Those results are shownin Fig.10 ~ Fig.13.

Fig6. Solar wind velocity variation (x) and neutron (Kiel) intensity variation (y)Fig10. Red: 2000~2002 Green: 2003~2005

Fig7. Solar wind velocity variation (x) and muon(N) intensity variation (y)The earlier 3 year’s slope seems to be slightly
steeper than later 3 years. It may be suggestingthat there may be some effect of solar activity.

SHORT TITLEof detection. We could not find any significant
North-South effect.

Acknowledgments

Fig11. Red: 2000~2002 Green: 2003~2005

For this analysis, the solar wind velocity used thedata of ACE level 2. Neutron intensity used thedata of the WDC observation station of Kiel. The
sunspot number used the data of "Solar Influences Data analysis Center". We wish to express
our gratitude to these organizations that havedisclosed their data.

Reference

Fig12. Red: 2000~2002 Green: 2003~2005

Fig13. Red: 2000~2002 Green: 2003~2005

Conclusion
So far it was pointed out often that there are some
temporal effect of solar wind velocity variation on
cosmic ray intensity variation due to the solarflare. Our present analysis suggests that the cosmic ray intensity variation decrease with the increase of solar wind velocity strongly. The effectof the solar activity also has been seen. We haveintroduced the cosmic ray muon data along withthe usual neutron monitors data in this analysis.
We could perform the detailed directional analysis using muon’s data utilizing the very large area

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